The present invention relates to a fuel gas supply system for a gas turbine and, in particular, to a system for controlling water level in a fuel gas saturator enabling a stable supply of a moisturized fuel gas to a gas turbine during all steady state and transient operating conditions of the fuel gas moisturization system.
Generally, a combined cycle power plant includes a gas turbine, a steam turbine, a heat recovery generator, a fuel superheater and a fuel gas saturator. Dry cold fuel gas is supplied to the fuel gas saturator, where the fuel gas is moisturized. The saturated fuel gas is then heated by the fuel superheater and is supplied to the gas turbine for combustion. The combustion reaction drives the turbine and a generator coupled to the gas turbine to produce electricity. The exhaust from the gas turbine enters a heat recovery generator (HRSG), which utilizes the heat from the exhaust gases to generate steam for use in the steam turbine and heats water for use in the fuel gas saturator and the fuel gas superheater. The steam generated in the heat recovery generator expands in the steam turbine, driving a generator for producing electricity. The water used in the fuel gas saturator requires heating because the fuel gas saturation process heats the gas as water is entrained in the fuel gas, the heat being provided by the exhaust gases from the gas turbine via heated saturation water.
Chemical process and Integrated Gasification Combined Cycle (IGCC) applications with saturators typically run at steady state load for long periods of time. The saturator column utilized by these processes and applications employ a fairly simple control based on constant water recirculation flow and single element closed loop level control via modulation of water make-up to drive the level signal error to zero. In this case, the level signal error equals the difference between the level setpoint and the measured water level. The controller increases or decreases the flow of water make-up based on the level signal error by opening or closing a valve in the water make-up stream. This type of control has been fully satisfactory for an IGCC that employs a diffusion combustor tolerant of variation in fuel supply heating value and temperature.
More demanding level control applications, such as boiler drum water level, add anticipatory control based on measurement of the flow entering and leaving the vessel to develop a feed forward signal to the water supply control valve. These are typically termed three element level controls, because water inlet flow, water outlet flow, and water level in the vessel are directly measured. An algorithm using those measurements is used to control the water supply.
Moisturized fuel supply to a Dry Low NOx gas turbine combustion system requires extremely tight control on the fuel saturation process. Natural gas fired combined cycles with Dry Low NOx (DLN) combustion systems impose strict requirements on the fuel saturation process due to tight fuel specification tolerances (variables such as heating value and temperature), frequent and rapid load changes and the absence of a backup fuel (which, if available, could be used to narrow the operating range required of the saturation system). Typically these DLN systems have at least two operating modes: one provides robust performance from initial ignition through early loading and the other provides optimized performance at base or high load. Minimizing system emissions is critical during operation at high load. Accordingly, operation of a finely tuned system for optimal performance at high load, i.e., at or near operating capacity, typically requires a low tolerance for variations in the fuel supply conditions.
A conventional three element level control applied to a fuel gas saturation column, as employed in a typical fuel gas moisturization system, requires measurement of the inlet fuel gas flow, make-up water flow, and exiting moisturized fuel gas flow. This is problematic since the make-up water flow rate is small in relation to the moisturized fuel gas flow rate, such that a small percentage error in the moisturized fuel gas flow rate measurement corresponds to a much larger percentage error in moisturized fuel gas moisture content.
In a preferred embodiment of this invention, three element level control of the water level in the sump of a fuel gas saturator column is provided without direct flow measurement of the moisturized fuel gas. In addition to improving the responsiveness of the level control relative to the responsiveness of a single element level controller, the present invention improves accuracy and reduces cost in relation to a conventional three element level control system that depends on direct measurement of the moisturized fuel gas flow rate.
Particularly, the present invention utilizes a relationship between the moisture content of the exiting fuel gas and measured inlet dry fuel gas flow rate to calculate a more accurate flow rate of water exiting with the moisturized gas. The moisture content of the moisturized fuel gas is calculated from measurements of moisturized fuel gas temperature (wet-and dry-bulb temperatures) and moisturized fuel gas pressure exiting the fuel gas saturator. The level of the water in the bottom of the saturation column is controlled using the more accurate, calculated flow rate of water exiting with the saturated fuel gas, the water inlet flow rate, and the level of water in the tank. Using these parameters, the controller adjusts the flow rate of the make-up water to ensure a stable level of water in the sump of the fuel gas saturator.
In another preferred embodiment, the saturated fuel gas moisture content is measured directly. The measurement is effected by using a humidity sensor or more precise composition measurement, such as gas chromatography. The measured saturated fuel gas moisture content is then used to calculate water outlet flow rate in combination with the measured dry fuel flow. The water outlet flow rate is used with measurements of water inlet flow rate and water level in the column sump to control the water level in the column sump.
In a preferred embodiment according to the present invention, there is provided a method for controlling the level of water in a fuel gas saturator sump comprising the steps of determining a proportion of the water in the moisturized fuel gas supplied by the gas saturator, determining the flow rate of dry fuel gas entering the gas saturator, calculating the flow rate of water in the moisturized fuel gas supplied by the gas saturator (without direct flow measurement of the moisturized fuel gas) and adjusting the flow of make-up water entering the fuel gas saturator in accordance with the determined flow rate of the water portion in the moisturized gas exiting the gas saturator to control the level of water in the fuel gas saturator thereby to maintain a stable level of water in the fuel gas saturator throughout all steady state and transient operating conditions.
In a further preferred embodiment according to the present invention, there is provided in a combined cycle system having a gas turbine and a steam turbine for generating electricity, a heat recovery system generator for recovering heat from the exhaust gases of the gas turbine and generating steam for use in the steam turbine and a fuel gas saturator for supplying moisturized fuel gas in the gas turbine, a method of controlling the level of water in the fuel gas saturator comprising the steps of determining a proportion of water in the moisturized fuel gas provided by the gas saturator and providing a first signal in response thereto, determining a flow rate of dry fuel gas entering the gas saturator and providing a second signal in response thereto, calculating a flow rate of water contained in the moisturized fuel gas provided by the gas saturator using the first and second signals and without direct flow measurement of the moisturized fuel gas and adjusting a flow of make-up water entering the fuel gas saturator in accordance with the calculated flow rate of the water contained in the moisturized gas exiting the gas saturator to control the level of the water in the fuel gas saturator, thereby to maintain a stable level of water in the fuel gas saturator throughout all steady state and transient operating conditions.
In a still further preferred embodiment according to the present invention, there is provided an apparatus for controlling the level of water in a fuel gas saturator sump comprising means for determining a proportion of water in the moisturized fuel gas supplied by the gas saturator without direct flow measurement of the moisturized fuel gas and providing a first signal in response thereto, a sensor for measuring a flow rate of a dry fuel gas entering the fuel gas saturator and providing a second signal in response thereto, means for determining a flow rate of water in the moisturized gas provided by the fuel gas saturator from the first and second signals and without direct flow measurement of the moisturized fuel gas and a controller for controlling the flow of make-up water entering the fuel gas saturator in accordance with the determined flow rate of water in the moisturized fuel gas supplied by the gas saturator, thereby controlling the level of the water in the fuel gas saturator.